Additional checks when evaluating the safety of an HTTP request, to avoid false negatives.

 * Check if the host is considered a safe redirect host.
 * Check if the host is another domain in a multisite installation.
 * Add a filter to control this.

This only occurs when the DNS resolution of a domain points elsewhere in an internal network, but only internally (and has its own public IP outside the network). This could be considered a bad configuration.

fixes #24646.



git-svn-id: https://core.svn.wordpress.org/trunk@24915 1a063a9b-81f0-0310-95a4-ce76da25c4cd